The goal of this research is to understand the mechanism of regulation of the methionine biosynthetic enzymes and the roles played by S-adenosylmethionine and some of its metabolites (spermidine, methylated nucleic acids and cyclopropane fatty acids) in cellular metabolism. These studies are centered around the isolation and characterization of mutants of E. coli K-12, which are constitutive for the methionine biosynthetic enzymes. One class of mutants of this type is also constitutive for S-adenosylmethionine synthetase and appear to involve the gene for an aporepressor. These and similar mutants will be genetically and biochemically studied in order to establish or reject the hypothetical role of the gene. Another class of mutants has reduced levels of S-adenosylmethionine synthetase and appear to be constitutive because they have a reduced pool of corepressor. We plan to isolate temperature sensitive mutants of this second class, which will hopefully have a complete lack of S-adenosylmethionine synthetase at the restrictive temperature. Using these and any other appropriate mutants, we will attempt to identify and map the structural gene (s) for S- adenosylmethionine synthetase and to identify the corepressor (s) for control of the structural genes of the methionine biosynthetic enzymes. We will also use mutants of this type to restrict the intracellular concentration of S-adenosylmethionine and to measure the effect of starvation for S-adenosylmethionine and/or its metabolites, on cellular processes such as protein and nucleic acid synthesis and gene expression.